The present invention relates to a high-performance composite semipermeable membrane for selectively transmitting and separating the components of liquid mixtures, a method for the production thereof and a method of removing harmful substances using same. With the composite semipermeable membrane obtained by means of the present invention it is possible, in particular, to recover drinking water at a high rate by permitting passage of silica and preventing deposition thereof at the membrane surface while selectively separating/removing the pollutants and trace quantities of harmful substances and their precursors, etc, contained in the raw water of water treatment plants.
In relation to the separation of mixtures, there are various techniques for removing materials (for example salts) dissolved in solvents (for example water) but, in recent years, membrane separation methods have come to be used as energy-saving and resource-efficient processes. The membranes in membrane separation methods are microfiltration membranes, ultrafiltration membranes and reverse osmosis membranes. Furthermore, recently, membranes positioned between reverse osmosis membranes and ultrafiltration membranes (loose RO membranes or NF membranes: nanofiltration membranes) have appeared and have come to be used. This technology makes it possible to obtain drinking water from, for example, sea water, salt/brackish water and water containing harmful substances and, moreover, the technology has also been employed for the production of ultra-pure water for industrial use, for waste water treatment and for the recovery of valuable materials, etc.
The majority of the composite semipermeable membranes currently marketed are of two kinds, namely those having a gel layer and an active layer of crosslinked polymer on top of a microporous support membrane and those having an active layer of polycondensed monomer on top of a microporous support membrane. Of these, composite semipermeable membranes formed by coating a microporous support membrane with an ultra-thin membrane layer of crosslinked polyamide obtained by a polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide are widely employed as reverse osmosis membranes of high permeability and selective separation characteristics.
However, the demand for practical semipermeable membranes for reverse osmosis is increasing year by year and, from the point of view of energy-saving, there is desired a semipermeable membrane with high water permeability where lower pressure operation is possible while still maintaining high solute removal properties. For example, from JP-A-64-56108 there is known a composite semipermeable membrane having good desalting properties and high water permeability in ultra-low pressure operation at 7.5 kg/cm2 (0.75 MPa), based on the presence of 4-chloroformylphthalic anhydride. However, even with this method the desalting properties and the water permeability are unsatisfactory in the case of super ultra-low pressure operation at around 0.3 MPa. Now, operation at a high recovery rate is also desirable, but with membranes where the percentage silica removal is high the silica concentration on the concentrate side increases rapidly, and deposition occurs at the membrane surface, so that a lowering of membrane performance results and stable operation and enhanced water quality cannot be expected.
In recent years, in water treatment plants using river water, and lake and swamp water, or the like, as the raw water, the formation of carcinogenic halogen-containing organic materials (trihalomethanes) has become a serious problem owing to the fact that in the water treatment plant there is carried out the chlorine sterilization treatment of the soluble organic matter (trihalomethane precursors) flowing-in from peat bogs and regions between mountains, etc. The most important of the trihalomethane precursors is humic acid, which comprises soluble organic matter of molecular weight ranging from several thousands to several tens of thousands. In the case of the ozone/active carbon treatment methods, the introduction of which is currently being investigated in water treatment plants, while the percentage removal at the time of the start of operation is high, when long-term operation is carried out the percentage removal falls rapidly. For this reason, frequent replacement of the active carbon is necessary. Furthermore, with contact oxidation methods, biological membrane methods and other such biological treatment methods, since soluble organic matter is formed at the end of the biological metabolism, there is the problem that sufficient removal cannot be carried out. In membrane separation methods, the microfiltration membrane and ultrafiltration membrane pore diameters are large and satisfactory removal of humic acid cannot be achieved. Furthermore, with reverse osmosis membranes, while the pore diameter is small and the percentage humic acid removal is high, the percentage silica removal is also high and consequently high-recovery operation using reverse osmosis membranes is difficult.
In order to resolve problems of the kind described above, the objective of the present invention lies in offering a composite semipermeable membrane having high solute removal properties and high water permeability, where high-recovery operation is possible.
In order to realize the aforesaid objective, the present invention relates to a composite semipermeable membrane which is characterized in that it is a composite semipermeable membrane in which there is formed by polycondensation, on a microporous support membrane, a crosslinked polyamide ultra-thin membrane layer from a polyfunctional amine, a polyfunctional acid halide and a polyfunctional acid anhydride halide, and the flow of water permeate at an operating pressure of 0.3 MPa, a temperature of 25xc2x0 C. and a pH of 6.5 lies in the range from 0.8 to 4.0 m3/m2.day and, furthermore, the percentage humic acid removal is at least 98% and, preferably, a composite semipermeable membrane characterized in that the carboxyl group concentration in the ultra-thin membrane layer analyzed using X-ray photoelectron spectroscopy (ESCA) is at least 0.02 but less than 0.06: a method for the production thereof; and a method of water purification using same.
The polyfunctional amine in the present invention is a mixed amine of aliphatic polyfunctional amine and aromatic polyfunctional amine, where the aliphatic polyfunctional amine is preferably a piperazine type amine or derivative thereof as represented by [Formula 1], examples being piperazine, 2,5-dimethylpiperazine, 2-methylpiperazine, 2,6-dimethylpiperazine, 2,3,5-trimethylpiperazine, 2,5-diethylpiperazine, 2,3,5-triethylpiperazine, 2-n-propylpiperazine, 2,5-di-n-butylpiperazine and the like, with in particular piperazine and 2,5-dimethylpiperazine being preferred. 
R1 to R8 are each selected from H, OH, COOH, SO3H, NH2 and C1 to C4 straight chain and cyclic, saturated and unsaturated, aliphatic groups.
Furthermore, the aromatic polyfunctional amine is not particularly restricted providing it has no less than two amino groups per molecule, and examples include m-phenylenediamine, p-phenylenediamine and 1,3,5-triaminobenzene, plus the N-alkyl derivatives thereof such as N,N-dimethyl-m-phenylenediamine, N,N-diethyl-m-phenylenediamine, N,N-dimethyl-p-phenylenediamine, N,N-diethyl-p-phenylenediamine and the like, with m-phenylenediamine and 1,3,5-triaminobenzene being particularly preferred.
The molar ratio of the aliphatic polyfunctional amine to the aromatic polyfunctional amine used in the present invention lies in the range from 40/60 to 95/5, more preferably from 70/30 to 90/10. If the aliphatic polyfunctional amine is less than 40 mol %, the flow of water permeate declines, while if it is greater than 95 mol % good selective separation characteristics are not obtained.
The polyfunctional acid halide is an acid halide having not less than two halocarbonyl groups per molecule and there are no particular restrictions thereon provided that a polyamide is formed by reaction with the aforesaid amine. As examples of the polyfunctional acid halides, there are the acid halides of 1,3,5-cyclohexanetricarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3-benzenedicarboxylic acid and 1,4-benzenedicarboxylic acid. In particular, from the point of view of cost, ease of procurement, ease of handling and ease of reaction, trimesoyl chloride, which is the acid halide of 1,3,5-benzenetricarboxylic acid, is preferred. Furthermore, the aforementioned polyfunctional acid halides can be used on their own or they may be employed as mixtures.
The polyfunctional acid anhydride halide referred to is a compound having one or more than one acid anhydride moiety and one or more than one halocarbonyl group per molecule, and examples include the carbonyl halides of benzoic anhydride and phthalic anhydride but, in terms of high water permeability and a suitable pore size for eliminating soluble organic materials, trimellitic anhydride halide and derivatives thereof as represented by the following general formula [Formula 2] are preferably used. 
X1 and X2 are each selected from C1 to C3 straight chain and cyclic, saturated and unsaturated, aliphatic groups, H, OH, COOH, SO3H, COF, COCl, COBr and COI, or they may form an acid anhydride,
X3 is selected from C1 to C3 straight chain and cyclic, saturated and unsaturated, aliphatic groups, H, OH, COOH, SO3H, COF, COCl, COBr and COI,
Y is selected from F, Cl, Br and I
As the material for the ultra-thin membrane in the composite semipermeable membrane of the present invention, there can be used a crosslinked or linear organic polymer. In order that the composite semipermeable membrane of the present invention manifests high separation performance, a polyamide, polyurethane, polyether, polyester, polyimide, cellulose ester or vinyl polymer is preferred as the polymer, with polyamides being particularly preferred from amongst these. A crosslinked polyamide obtained by the reaction between a polyfunctional amine, a polyfunctional acid halide and a polyfunctional acid anhydride halide is further preferred.
The molar ratio of the polyfunctional acid halide to the polyfunctional acid anhydride halide used in the present invention is important in terms of obtaining a composite semipermeable membrane with the opposing properties of high water permeability and selective separation. The feed molar ratio of polyfunctional acid halide to polyfunctional acid anhydride halide is preferably from 75/25 to 15/85, and more preferably 65/35 to 35/65. If the molar proportion of polyfunctional acid anhydride halide falls below 25, then the flow of water permeate is reduced, while if it exceeds 85 then good selective separation is no longer obtained.
A preferred example of the microporous support membrane is a polysulfone support membrane which has been reinforced by a fabric. The microporous support membrane is a layer which essentially does not possess separation properties but is used to confer mechanical strength on the ultra-thin membrane layer which possesses the separation properties. A support membrane structure is preferred which has uniform microfine pores or which has microfine pores which gradually become larger from one side to the other and where the size of these microfine pores at the surface on this one side is no more than 100 nm. This microporous support membrane can be selected from various types of commercially-available material such as xe2x80x9cMillipore Filter VSWPxe2x80x9d (commercial name) made by the Millipore Co. and xe2x80x9cUltrafilter UK10xe2x80x9d (commercial name) made by the Toyo Roshi Co., but normally it may be produced by the method described in the xe2x80x9cOffice of Saline Water Research and Development Progress Reportxe2x80x9d No. 359 (1968). The material used is normally a homopolymer such as polysulfone, cellulose acetate, cellulose nitrate or polyvinyl chloride, or a blend thereof, but the use of a polysulfone of high chemical, mechanical and thermal stability is preferred. For example, a dimethylformamide (DMF) solution of a polysulfone is cast to a given thickness onto a tightly-woven polyester fabric or onto nonwoven material and, by wet coagulation thereof in an aqueous solution containing, for example, 0.5 wt % sodium dodecylsulphate and 2 wt % DMF, a microporous support membrane having microfine pores of diameter no more than a few tens of nm over the greater part of the surface is obtained.
The carboxyl group concentration is the amount of carboxyl groups (moles) in terms of the total amount of carbon (moles) in the ultra-thin membrane layer, and it is given by the formula [3].                               Carboxyl          ⁢                      xe2x80x83                    ⁢          group          ⁢                      xe2x80x83                    ⁢          concentration                =                              amount            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            carboxyl            ⁢                          xe2x80x83                        ⁢            groups            ⁢                          xe2x80x83                        ⁢                          (              moles              )                                            total            ⁢                          xe2x80x83                        ⁢            carbon            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            ultra            ⁢                          xe2x80x83                        ⁢                          -                        ⁢                          xe2x80x83                        ⁢            thin            ⁢                          xe2x80x83                        ⁢            membrane            ⁢                          xe2x80x83                        ⁢            layer            ⁢                          xe2x80x83                        ⁢                          (              moles              )                                                          [Formula  3]            
The carboxyl group concentration can be determined using an X-ray photoelectron spectroscopy method (ESCA), employing the gas phase chemical modification method exemplified in Journal of Polymer Science Vol. 26, 559-572 (1988) and in Nihon Setchaku Gakkai-shi (J. Adhesion Soc. Japan) Vol. 27, No.4 (1991).
Below, a method of measuring the carboxyl group concentration is explained. As a labelling reagent for the carboxyl groups, trifluoroethanol is used. The sample is subjected to gas phase chemical modification by means of the labelling reagent and, from the ESCA spectrum of a polyacrylic acid standard sample which has undergone gas phase chemical modification at the same time, the percentage reaction (r) of the labelling reagent and the percentage reaction residue remaining (m) is obtained. Next, the peak area [F1s] of the F1s peak (fluorine 1s orbital peak) following reaction between the sample and the labelling reagent is obtained. Again, by elemental analysis, the area intensity [C1s] of the C1s peak (carbon 1s orbital peak) is obtained.
The measurement conditions are as follows:
excitation X-rays: Mg Kxcex1, 1,2 line (1253.6 eV)
X-ray output: 8 kV 30 mV
take-off angle: 90xc2x0
In the data processing, the C1s peak position for the neutral carbon (CHx) is set to 284.6 eV.
By introducing the area intensities [F1s] and [C1s] obtained as described above into Formula [4] given in Journal of Polymer Science Vol.26 559-572 (1988), it is possible to obtain the carboxyl group concentration.                               R          COOH                =                              [            F1s            ]                                              (                                                3                  ⁢                                      xe2x80x83                                    ⁢                                                            k                      F1s                                        ⁡                                          [                      C1s                      ]                                                                      -                                                      (                                          2                      +                                              13                        ⁢                                                  xe2x80x83                                                ⁢                        m                                                              )                                    ⁡                                      [                    F1s                    ]                                                              )                        ⁢                          xe2x80x83                        ⁢            r                                              Formula  [4]            
RCOOH: carboxyl group concentration, [F1s]: area intensity of the fluorine 1s orbital peak, kF1s: sensitivity correction for the fluorine 1s orbital peak, r: percentage reaction of the labelling reagent, [C1s]: area intensity of the carbon 1s orbital peak, m: residual percentage of the reaction residue
When the carboxyl group concentration is high, there is an increase in the carboxyl group terminals in the membrane and the water permeability rises but the crosslink density is decreased and the harmful material removal properties are lowered. Conversely, when the carboxyl group concentration is low, there is a reduction in unreacted terminals and the crosslink density is increased, so the water permeability is lowered. Hence, the carboxyl group concentration should be at least 0.02 but less than 0.06, and preferably at least 0.02 and no more than 0.04. Now, the thickness of the ultra-thin membrane layer in the composite semipermeable membrane should be from 1 nm to 300 nm, and preferably from 1 nm to 100 nm. If the thickness of the ultra-thin membrane layer is too low, the occurrence of defects increases at the time of membrane production, damage readily occurs at the time of handling, and when pressure is applied faults are produced and a lowering of the removal rate is brought about. Furthermore, if the ultra-thin membrane layer is too thick, the permeability rate/coefficient is reduced and adequate permeation is not obtained.
The roughly spherical projections at the surface of the ultra-thin membrane layer are small projections of height 1 to 800 nm and diameter 1 to 500 nm, and they can be observed from a scanning electron micrograph or a transmission electron micrograph of the ultra-thin membrane layer surface or of a cross-section. Furthermore, by analysis of the electron micrograph, it is possible to determine the size of the individual roughly spherical projections and the distribution thereof. For example, in the case of a scanning electron microscope surface photomicrograph, the surface of the membrane sample is thinly coated with platinum or quaternized ruthenium, preferably quaternized ruthenium, and observation made at an acceleration voltage of 1 to 6 kV with a high resolution field emission scanning electron microscope (UHR-FE-SEM). As the high resolution field emission scanning electron microscope there can be used, for example, the model S-900 electron microscope made by Hitachi Ltd., or the like. The observation magnification is preferably 5,000 to 100,000 times, and in order to determine the size distribution of the roughly spherical projections a magnification of 10,000 to 50,000 times is preferred. Taking into account the magnification used, the size of the roughly spherical projections can be directly measured from the electron micrograph obtained by using a ruler or the like.
The roughly spherical projections in the present invention are the small projections of roughly spherical shape covering the ultra-thin membrane layer as seen in the electron micrograph of the composite semipermeable membrane surface, and the ratio of the major/minor axes thereof can be determined by the following method. As an example, a square of side 10 cm is drawn on a scanning electron micrograph of the membrane surface obtained at a magnification of 20,000. Next, with a ruler, the major and minor axes of the roughly spherical projections in the square are measured. In this way, it is possible to measure the major and minor axes of all the roughly spherical projections in the square and obtain the distribution thereof. Now, even where half or more of the roughly spherical projections in the aforesaid electron micrograph are obscured or where calculation is performed excluding those roughly spherical projections which, in terms of the shadowing, are below the observable limits, no problem arises, and the lower limit of the major axis is not particularly restricted but, preferably, it is about 50 nm. Furthermore, when only the upper semicircle of a roughly spherical projection can be seen in the aforesaid micrograph, by estimating the lower semicircle the roughly spherical projection can be traced out, the major and minor axes then measured and the ratio of the major/minor axes determined.
Furthermore, the ratio of the major/minor axes of the roughly spherical projections and the distribution thereof can also be obtained by image processing, by inputting the electron micrograph, or a trace of the roughly spherical projections obtained from the electron micrograph, into a computer. For example, calculation can be carried out with the image processing software xe2x80x9cPxe2x80x2-Analyzerxe2x80x9d using a xe2x80x9cPIAS-IVxe2x80x9d device produced by PIAS K.K. Furthermore, it is possible to calculate the distribution from all the roughly spherical projections recorded on the electron micrograph by means of such methods.
Roughly spherical projections (also referred to as pleated structures) have already been observed in composite membranes produced by the interfacial polycondensation method, and it is reported that the flow of water permeate is increased by the increase in surface area (Mutsuo Kawasaki, Takeshi Sasaki and Masahiko Hirose, Maku, 22(5), 257-263 (1997)) but, rather than being roughly spherical, the shapes thereof were kombu seaweed-shaped (that is, having a slightly wrinkled or pleated structure). As a result of considerable investigation, it has been discovered that roughly spherical projections where the ratio of the major axis/minor axis of these roughly spherical projections in the ultra-thin membrane layer is made to lie in the range 1.0 to 2.0 by adjustment of the monomer used and the film-forming conditions, are preferred in terms of enhancing the separation properties in the case of a composite semipermeable membrane used at low pressure. With regard to the distribution thereof, it is desirable that the roughly spherical projections where the ratio of the major/minor axes is 1.0 to 2.0 comprise at least 70%, and preferably at least 80%, of the roughly spherical projections as a whole. Where this distribution is less than 70%, the ultra-thin membrane layer is non-uniform so it is possible that the desired membrane properties will not be obtained or that variations in membrane performance are increased. Furthermore, where the ratio of the major/minor axes of the roughly spherical projections is more than 2.0, then, rather than roughly spherical projections, they become tubular or kombu seaweed-shaped. Contamination readily builds up in the gaps there-between and problems may arise in the pressure resistance when used at high pressure.
The provision of the ultra-thin membrane layer in the present invention can be carried out by the method of coating polymer, the method of further crosslinking polymer which has been coated, the method of polymerising monomer at the membrane surface of the microporous support membrane, or the method of carrying out interfacial polycondensation at the membrane surface of the microporous support membrane. In particular, the ultra-thin membrane layer referred to in the present invention, with projections at the surface the tips of which are roughly circular, can be obtained by the method of interfacial polycondensation at the membrane surface of the microporous support membrane. In such circumstances, it is possible to control the size of the roughly spherical projections by altering the solution concentrations or the additives used in the interfacial polycondensation. Again, using a hydrocarbon with 7 or more carbons as the water-immiscible solvent is effective for obtaining the ultra-thin membrane layer of the present invention. Next, the method of producing the composite semipermeable membrane is explained.
The ultra-thin membrane layer in the composite semipermeable membrane, which essentially possesses the separating properties, is formed by carrying out reaction on the aforesaid microporous support membrane between an aqueous solution containing the aforesaid amine and a water-immiscible organic solvent solution containing the aforesaid polyfunctional acid halide together with polyfunctional acid anhydride halide.
The molar ratio of the aliphatic polyfunctional amine to the aromatic polyfunctional amine used in the present invention is, as stated above, from 40/60 to 95/5, and more preferably from 70/30 to 90/10, and the concentration of the amine in the aqueous solution of mixed amine is from 0.1 to 20 wt %, preferably 0.5 to 15 wt %. Again, providing they do not obstruct the reaction between the amine compound and the polyfunctional acid halide plus the polyfunctional acid anhydride halide jointly present therewith, there may also optionally be present, in the aqueous solution and in the organic solvent solution, compounds such as an acylation catalyst, polar solvent, acid scavenger, surfactant, antioxidant and the like.
With regard to the covering of the microporous support membrane surface with the aqueous amine solution, said aqueous solution should be applied uniformly and continuously to the surface and this may be carried out by known application means such as, for example, the method of coating the aqueous solution onto the surface of the microporous. Next, the method of producing the composite semipermeable membrane is explained.
The ultra-thin membrane layer in the composite semipermeable membrane, which essentially possesses the separating properties, is formed by carrying out reaction on the aforesaid microporous support membrane between an aqueous solution containing the aforesaid amine and a water-immiscible organic solvent solution containing the aforesaid polyfunctional acid halide together with polyfunctional acid anhydride halide.
The molar ratio of the aliphatic polyfunctional amine to the aromatic polyfunctional amine used in the present invention is, as stated above, from 40/60 to 95/5, and more preferably from 70/30 to 90/10, and the concentration of the amine in the aqueous solution of mixed amine is from 0.1 to 20 wt %., preferably 0.5 to 15 wt %. Again, providing they do not obstruct the reaction between the amine compound and the polyfunctional acid halide plus the polyfunctional acid anhydride halide jointly present therewith, there may also optionally be present, in the aqueous solution and in the organic solvent solution, compounds such as an acylation catalyst, polar solvent, acid scavenger, surfactant, antioxidant and the like.
With regard to the covering of the microporous support membrane surface with the aqueous amine solution, said aqueous solution should be applied uniformly and continuously to the surface and this may be carried out by known application means such as, for example, the method of coating the aqueous solution onto the surface of the microporous support membrane or the method of immersing the microporous support membrane in said aqueous solution.
Next, the excess applied aqueous solution is eliminated by means of a liquid removal stage. The method used for this may be, for example, by holding the membrane surface vertically and allowing the liquid to run off naturally. Having removed the excess liquid, the membrane surface may then be dried, to eliminate some or all of the water from the aqueous solution. Next, there is applied the organic solvent solution of the polyfunctional acid halide in which the aforesaid polyfunctional acid anhydride halide is jointly present, so that there is formed by reaction an ultra-thin layer of crosslinked polyamide. The concentration of the polyfunctional acid halide is not particularly restricted, but if it is too low there is insufficient formation of the ultra-thin membrane which constitutes the active layer and there is the possibility of defects being produced, while if the concentration is too great this is disadvantageous in terms of cost. Hence, from about 0.01 to about 1.0 wt % is preferred. The method of effecting contact between said aqueous amine solution and the polyfunctional acid halide along with the jointly present polyfunctional acid anhydride halide may be the same as the method used for coating the microporous support membrane with the aqueous amine solution. Furthermore, the elimination of the organic solvent following the reaction can be carried out by the method of holding the membrane vertically, so that the excess non-polar solvent drains away naturally.
The organic solvent needs to be water-immiscible and, furthermore, needs to be capable of dissolving the polyfunctional acid halide, but without damaging the microporous support membrane and, providing that a crosslinked polymer can be formed by reaction, any such solvent may be used. Typical examples are liquid hydrocarbons and halo-hydrocarbons such as trichlorotrifluoroethane. However, taking into account potential damage to the ozone layer, ready availability, ease of handling and handling safety, octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, heptadecane, hexadecane and the like, cyclooctane, ethylcyclohexane, 1-octene, 1-decene and other such individual solvents or mixtures thereof are preferably used.
The shape of the membrane may be either that of a hollow fibre or a flat membrane. The composite semipermeable membrane of the present invention can be used incorporated into a spiral, tubular or plate-and-frame module, but the invention is not restricted to these usage forms.
Using the composite semipermeable membrane of the present invention, harmful substances and the precursors thereof present in raw water can be removed at an operating pressure of 0.1 to 3.0 MPa. If the operating pressure is lowered, the capacity of the pump used is reduced and power consumption reduced but, on the other hand, the membrane tends to become blocked and the flow of water permeate is reduced. Conversely, if the operating pressure is increased, the power consumption is raised for the aforesaid reason, and the flow of water permeate is increased. Consequently, the operating pressure range is desirably 0.1 to 3.0 MPa, preferably 0.2 to 2.0 MPa and particularly 0.2 to 1.0 MPa. If the flow of water permeate is high, blockage is brought about due to fouling of the membrane face, while if it is low, costs are increased, so the range for the flow of water permeate is desirably 0.8 to 4.0 m3/m2.day and preferably 0.85 to 2.5 m3/m2.day in terms of maintaining a stable amount of water. Again, in order to reduce the water production costs and recover the supplied water efficiently, the recovery should be at least 80%, preferably at least 85% and more preferably at least 90%.
As examples of the harmful substances referred to here, there are trihalomethane precursors. Trihalomethane precursors produce carcinogenic trihalomethanes in the chlorine sterilization at a water purification plant. These trihalomethane precursors include humic acid and fulvic acid. In particular, the amount of trihalomethane produced is considerable with humic acid, and the percentage removal thereof should be at least 98% and more preferably at least 99%.
By using the composite semipermeable membrane of the present a invention, in the case of a silica concentration of 30 ppm as SiO2 (which is the average value across Japan) the silica is allowed to pass and so it is possible to prevent the production of silica scale. With a membrane of low silica passage, when operation is conducted at a high percentage recovery the silica is deposited on the membrane face and blocks the membrane surface, so the problem arises that a high level of water passage is no longer obtained. Hence, the silica passage should be at least 55% and more preferably at least 65%.
By employing the means as described above, it is possible to obtain a composite semipermeable membrane having a high humic acid removal property and high water permeability not achievable in conventional microfiltration membranes, ultrafiltration membranes, reverse osmosis membranes and the like, and operation at a high percentage recovery is possible due to the high silica permeability.